This invention relates to black thermal control films, and to microwave antennas having radomes utilizing such films.
Many types of microwave devices have antennas which transmit and/or receive ("transceive") microwave energy through free space. In one application, transceivers on communications satellites receive relatively faint microwave signals transmitted from earth stations, amplify those signals, and retransmit the high-power microwave signals back to earth stations at other locations. These devices and their antennas are designed to operate for years without interruption or degradation of service.
The microwave transceiver antennas used on spacecraft must be protected against several types of damage which could adversely affect their operation and operating lifetimes. They must be protected against overheating from the sun's energy when the antennas are facing the sun. They must also be protected against electrostatic charging and accumulation of dust-like debris on their surfaces.
The conventional approach to protecting antennas utilizes a radome or a protective film. The radome is a cover that fits over the antenna and through which the microwave transceived energy passes. For the spacecraft applications, the radome must have the protective functions discussed above, as well as have a low insertion loss in the microwave energy that passes through the radome material. The radome also must be light in weight, because of the high cost of lifting loads to orbit. The requirement of RF (radio frequency) transparency imposes a significant constraint on the radome, because, to some extent, the ability to prevent the buildup of electrical static charge and RF transparency are apparently incompatible. Ideally, the sunshield would be electrically conductive to bleed static charges, but be a dielectric for RF transparency.
Several approaches are known in an attempt to satisfy the shielding requirements for spacecraft antennas. In one, a polyimide film has a thin layer of a conductive material such as germanium on one side to aid in the dissipation of electrostatic charge. This material is RF transparent, but it has minimal electrostatic charge dissipation capability. Additionally, the germanium layer does not significantly contribute to the thermal properties of the freestanding film, sometimes termed a "sunshield". No sunshield film currently exists with a bulk resistivity sufficiently low to dissipate static electricity. As a result of this, bulk static discharges occur from high velocity electrons that lodge into this bulk film. All current films use surface coatings to minimize static discharge. This type of structure only dissipates surface charges and therefore bulk discharges are allowed to occur. In another approach, particles of a conductive pigment are dispersed through the film. The particles aid in dissipating the bulk charges by particle-to-particle conduction. However, conductive loadings in bulk films tend to block the transmission of RF signals. In another approach, an electrically conductive paint may be used, but such paints are heavy.
There is a need for an improved radome and film material that provides the necessary thermal control, meets microwave transmission and electrostatic discharge requirements, and also is light in weight. The present invention fulfills this need, and further provides related advantages.